Nitrogen dioxide and dinitrogen tetroxide form a temperature-dependent equilibrium…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
Nitrogen dioxide and dinitrogen tetroxide form a temperature-dependent equilibrium described by
2NO2(g)⇌N2O4(g)
A mixture initially at equilibrium at 298 K in a rigid 2.0 L container has the following concentrations:
[NO2]eq=0.40mol L−1 and [N2O4]eq=0.10mol L−1.
Sudden experimental manipulations are performed on this equilibrium system. For each manipulation listed below, answer the questions that follow. Treat each change as occurring independently, i.e., the system is reset to its original equilibrium state before the next manipulation is applied.
(a) The volume of the container is instantaneously reduced from 2.0 L to 1.0 L at constant temperature.
(i) Immediately after the volume change, has the numerical value of Qc increased, decreased, or remained the same? Explain without doing a full calculation.
(ii) State the sign of ΔG for the forward direction at the moment just after compression and justify your answer.
(iii) Predict the net direction in which the reaction will proceed to reach a new equilibrium.
(b) An inert gas is injected until the total pressure doubles, while the volume remains fixed at 2.0 L and the temperature is constant.
(i) Explain whether Qc changes as a direct result of adding the inert gas.
(ii) Predict and justify any shift in the position of equilibrium, if one occurs.
(c) A brief pulse of additional 0.020mol of NO2(g) is injected into the container at constant volume and temperature.
(i) Qualitatively compare the new value of Qc to Kc immediately after injection.
(ii) Describe how the concentrations of NO2 and N2O4 will change over time until equilibrium is re-established.
(iii) Sketch or describe the expected shape of the concentration–time curve for N2O4 over the interval from injection to the attainment of the new equilibrium.
(d) Summarize, in one or two sentences, how the concepts of Q, K, and ΔG together rationalize the spontaneous shifts observed in parts (a)–(c).